This invention relates to a control system for the automatic shift of a step change gear transmission.
Presently known step change gear transmissions for motor vehicles are shifted automatically in dependence on speed, preferably during speed reduction through gearing, and on load conditions as well as the various positions of the fuel controlling gas pedal, such as idling and partial load or full load (kick-down).
It is also known that certain gear ratios of a transmission may be excluded from the shift sequence under selective driver control, for example, under downhill driving conditions. Besides the standard selector control positions for forward travel D, neutral position N and reverse travel R, such selector controls have additional control positions I, II and III, whereby automatic shift of the gearing is correspondingly limited to the first gear ratio, the second ratio or the third gear ratio, as described for example in the brochure, "ZF Automatic Gearings HP 500, Description, Operation and Maintenance F 1/12 ZFF 792006". Such a system offers to the driver the ability to automatically operate a step change gear transmission with relatively fewer gear steps.
In the case of certain multigear-automatic transmissions, not only higher but also lower gear ratios are excluded from the automatic gear shift operation by corresponding displacement of two selector levers to idling and partial travel load positions, respectively. Furthermore, in certain automatic step change gear transmissions, the shift points are changed by means of a preselection control so as to provide more economical performance.
All of such prior art control systems, especially automatic shift controls for multi-speed step change gears, are unable to readily adapt the drive train from the engine to the performance characteristics of the engine so as to achieve favorable fuel economy. In the case of the usual automatic gear control system having relatively few speed ratios, the duration of drive at each speed ratio is prolonged because of the relatively high reduction gear ratios and the wide gear ratio differences between steps thereby reducing the frequency of shift. A different problem arises, however, in the case of multi-speed, automatic gear transmissions with narrow differences between gear ratio steps, as is required for fuel saving operation.
Therefore, it is an object of the present invention to provide an automatic gear shift control system which takes into consideration the small gear ratio differences between shift points of a multi-speed, automatic gear transmission without excessive shifting frequency as a result of the relatively large number of gear ratios involved.